In mobile telephony, radio paths are not optimized as they are in line of sight (LOS) microwave and satellite communications that have fixed sites selected for optimal propagation. Mobile systems introduce additional variables including radio motion, obstacles, and a third dimension. Indeed, a mobile radio terminal may be moving at high speeds in an automobile. But even if the terminal is temporarily fixed, that location may be anywhere within a serving area of interest. A stationary terminal user moving a hand-held terminal during a conversation introduces "micromotion" as well. The end result is that the characteristics of the radio communications path are constantly changing.
Multi-path propagation therefore is the rule in mobile telephony. Consider the simplified multi-path pictorial model in FIG. 1. Multiple rays P.sub.1, P.sub.2, P.sub.3, from radio 10 reach the receive antenna of the mobile radio 12 each with its own delay. Moreover, RF energy arrives on the receive antenna reflected off the sides of buildings, streets, lakes, the atmosphere, and so on as well as diffracted by both edge-type structures like building corners and rounded obstacles like water tanks and hilltops.
Because the same signal arrives over several paths, each with a different electrical length, the phase of the signal over each path is different, resulting in complicated constructive and destructive amplitude fading. Fades of 20 dB are common, and even 30 dB fades can be expected.
Analog FM mobile radio receivers employ FM discriminators to convert a changing frequency or phase into a corresponding appropriate voltage signal to drive a speaker. To make this kind of conversion, signal discriminators are sensitive to changes in slope and phase of the received signal. However, as a mobile unit moves, the received signal fluctuates in both amplitude and phase. Multi-path fading in particular causes fluctuations in phase that generate short bursts of noise. These noise bursts at the output of the FM discriminator are sometimes referred to as random FM noise or "click noise."
To understand the click noise phenomena, consider a received signal as a phasor or vector Re.sup.j.spsp..phi.R in the complex domain having an amplitude R and a phase angle .phi..sub.R Vector R has two vector components: re.sup.j.spsp..phi.R represents the fading signal but nevertheless desired signal and ne.sup.j.spsp..phi.R corresponds to the additive noise. Since the fading changes very slowly compared with the noise, the resulting vector R appears to rotate rapidly about the point where the two vector components intersect. Occasionally vector R sweeps around the origin when r&lt;n which causes the phase angle .phi..sub.R to increase or decrease by 2.pi. radians or 360.degree. C. Such dramatic and sudden phase increases/decreases produce spike-type impulses that correspond to "clicks."
An example of a click output from a discriminator is shown in FIG. 2. When this kind of click impulse is applied to one or more low pass filters, such as is found in the audio processing portion of FM receivers, a corresponding but wider impulse response is produced as shown in FIG. 3. In addition, a "ringing" waveform that precedes and follows the relatively narrow click pulse to effectively substantially enhance the distorting effect of the click. In physical terms, click noise is generated at the FM discriminator output when the radio experiences a fade that causes a rapid phase change of 2.pi./360.degree..
The problem is how to detect and minimize the impact of these undesirable clicks. One crude approach might be to use a simple high pass filter followed by a gain reduction stage to minimize the audible effect of the click. But unless the high pass filter is tuned to the shape of the click or to the frequency characteristics of the desired received signal, it is difficult to detect and attenuate just the click itself. As a result, the desired signal may be attenuated rather than the click. Moreover, the click may not be sufficiently attenuated to minimize its undesirable effects.
Even assuming a click can be accurately detected, removing the click using a squelch or noise blanking circuit destroys desired information present along with the detected click. This problem is particularly troublesome if the desired information includes both voice and sub-audible control signals. While a loss of voice may not be noticed or its absence tolerated, such a loss may not be as well tolerated when the controls signals include low speed digital data.